Investments - Fair Value and Unrealized Losses (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 securities	Dec. 31, 2011 securities
Fair value
Less than 12 months, Fair Value	$ 215,895	$ 302,657
12 Months or More, Fair value	134,043	125,089
Total, Fair value	349,938	427,746
Unrealized losses
Less than 12 months, Unrealized losses	(3,172)	(7,855)
12 Months or More, Unrealized losses	(6,978)	(22,830)
Total, Unrealized losses	(10,150)	(30,685)
Number of securities in loss position (in securities)	39	62
12 Months or More, number of securities (in securities)	8	8
U.S. agency bonds – mortgage-backed [Member]
Fair value
Less than 12 Months, Fair value	149,095	30,447
12 Months or More, Fair value	0	0
Total, Fair value	149,095	30,447
Unrealized losses
Less than 12 Months, Unrealized losses	(1,335)	(75)
12 Months or More, Unrealized losses	0	0
Total, Unrealized losses	(1,335)	(75)
Non-U.S. government bonds [Member]
Fair value
Less than 12 Months, Fair value	27,610	43,629
12 Months or More, Fair value	0	0
Total, Fair value	27,610	43,629
Unrealized losses
Less than 12 Months, Unrealized losses	(285)	(293)
12 Months or More, Unrealized losses	0	0
Total, Unrealized losses	(285)	(293)
Corporate bonds [Member]
Fair value
Less than 12 Months, Fair value	37,480	227,367
12 Months or More, Fair value	134,043	125,089
Total, Fair value	171,523	352,456
Unrealized losses
Less than 12 Months, Unrealized losses	(1,497)	(7,406)
12 Months or More, Unrealized losses	(6,978)	(22,830)
Total, Unrealized losses	(8,475)	(30,236)
Available-for-sale securities [Member]
Fair value
Less than 12 Months, Fair value	214,185	301,443
12 Months or More, Fair value	134,043	125,089
Total, Fair value	348,228	426,532
Unrealized losses
Less than 12 Months, Unrealized losses	(3,117)	(7,774)
12 Months or More, Unrealized losses	(6,978)	(22,830)
Total, Unrealized losses	(10,095)	(30,604)
Other investments [Member]
Fair value
Less than 12 months, Fair Value	1,710	1,214
12 Months or More, Fair value	0	0
Total, Fair value	1,710	1,214
Unrealized losses
Less than 12 months, Unrealized losses	(55)	(81)
12 Months or More, Unrealized losses	0	0
Total, Unrealized losses	$ (55)	$ (81)